Most water contains alkaline-earth metal cations, such as barium, strontium, calcium and magnesium, and anions, such as sulfate, bicarbonate, carbonate, oxalate, phosphate, silicate and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. For example, when the barium ion and sulfate ion exceed the solubility of the barium sulfate reaction product, a solid phase of barium sulfate will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure or temperature and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution.
As these reaction products precipitate on the surfaces of the water-carrying or water-containing system, they form adherent deposits or scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, in production systems for oil and gas, in pulp and paper mill systems, and in other systems, causing delays and shutdowns for cleaning and removal.
Once through and recirculating, cooling water systems are subject to the formation of scale deposits. Waterside problems encountered in boilers and steam systems include the formation of scale and other deposits, corrosion and foam. Scale and other deposits on heat-transfer surfaces can cause loss of the thermal efficiency of the boiler and can make the temperature of the boiler metal increase. Under scaling conditions, temperatures may go high enough to lead to failure of the metal due to overheating.
Barium and strontium sulfate scale deposits present a unique and sometimes "unsolvable" problem. Under most conditions, these sulfates are considerably less soluble in all solvents than any of the other commonly encountered scale-forming compounds. It is generally acknowledged that barium sulfate scale is almost impossible to remove by chemical means. Consequently, barium sulfate must be removed mechanically or the equipment, pipes, etc., containing the deposit must be discarded.
The incidence of barium sulfate scale is worldwide, and it occurs principally in systems handling subsurface waters. The barium sulfate scale problem is of particular concern to the petroleum industry since increasing volumes of water are produced with petroleum and more petroleum is produced by the water-flooding method of secondary recovery. The scale may occur in many different places, including production tubing, wellbore perforations, the area near the wellbore, gathering lines, meters, valves and in other production equipment.
Deposition of scale in production facilities and formation channels is a well-known source of problems in oil recovery. Barium sulfate scale is particularly troublesome when sulphate-rich seawater is used as an injection fluid in oil wells whose formation water is rich in barium ions. This scale causes severe problems in U.S. oil fields and older North Sea oil fields. Scaling of this nature is expected to occur during advanced production stages in other North Sea fields particularly after seawater breakthrough has taken place.
Barium sulfate scale may also form within subterranean formations such as in disposal wells. Scales and deposits can be formed to such an extent that the permeability of the formation is impaired resulting in lower flow rates, higher pump pressures, and ultimately abandonment of the well.
U.S. Pat. No. 5,093,020 which issued to Paul et al. teaches a method and composition for removing an alkaline earth metal sulfate scale deposits from wellbores and equipment used in the production of hydrocarbonaceous fluids from a reservoir or formation. The composition comprises an aqueous solution having a pH of about 8 to about 14, an EDTA or DTPA chelant, and a catalyst or synergist. Preferred chelants comprise diethylenetriaminepentaacetic acid (DTPA) or ethylenediaminetetraacetic acid (EDTA) or alkali salts thereof.
Although this method is effective in removing alkaline earth metal sulfate scale deposits from wellbores, precise placement of the composition into a desired interval of the wellbore could be improved. This is particularly true when diversion of this high density composition over an extensive interval is required in a hydrocarbonaceous fluid producing formation. If several feet of wellbore have been perforated for production, and if the perforated interval contains sections of high permeability (greater than 10 to 20 md), it may be difficult to effectively treat the entire zone with this high density composition at low surface injection rates, i.e. about 1 to about 2 BPM.
Therefore, what is needed is a method that would permit distribution of a high density alkaline earth metal sulfate scale solubilizing composition over an extended perforated productive interval of a formation or reservoir.